Various systems have been used for fractionating gaseous mixtures in which relatively adsorbent or similar material is employed to remove one or more components from a gaseous mixture. The overall effectiveness of such a material is reduced as it becomes laden with the component or components that are to be removed from the mixture, and therefore requires periodic regeneration.
Adsorbent gas fractionators usually involve the use of two adsorbent beds, one of which is being regenerated (i.e., desorbed) while the other is on the adsorptive cycle. Typically, the gas to be fractionated is passed through one sorbent bed in one direction in the adsorption cycle, and then, after a predetermined time interval, when the sorbent can be expected to have adsorbed so much of the gas being removed that there is a danger that the required low concentration of that gas in the effluent will not be met, the influent gas is switched to the other adsorbent bed, and the spent adsorbent bed is regenerated by heating and/or by evacuation and/or by passing purge effluent gas therethrough, usually in counterflow, and at a reduced pressure.
Adsorbent gas fractionators on the market today are of two general types, a heat-reactivatable type, in which heat is applied to regenerate the spent sorbent at the conclusion of the adsorbent cycle, and a heatless or pressure swing type, in which heat is not applied to regenerate the spent sorbent at the conclusion of the adsorbent cycle, but in which a purge flow of pure gas, usually effluent gas from the bed on the adsorption cycle, is passed through the spent bed at a lower pressure.
The present invention is directed to pressure swing or heatless type of adsorbent gas fractionation systems whereby a gaseous mixture, at elevated pressure, first passes through an adsorbent material after which a small fraction of the purified stream, at reduced pressure is flowed through the contaminant laden material desorbing the contaminant and carrying it away.
Conventional pressure swing or heatless processes utilize a pair of adsorbent beds, i.e., one to purify the pressurized gaseous mixture while the other is being purged of the contaminants by a metered amount of the purified gas which has been expanded to a lower pressure. Control systems and automatic valves are typically used to reverse the role of the two sorbent beds at intervals.
Some examples of conventional pressure swing or heatless adsorbent fractionator processes and systems are set forth in U.S. Pat. Nos. 4,205,967 (Sandman et al.), which issued Jun. 3, 1980; 4,832,711 (Christel, Jr. et al.), which issued May 23, 1989; 4,954,146 (Garrett et al.), which issued Sep. 4, 1990; 4,494,966 (Umeki), which issued Jan. 22, 1985; 4,329,158 (Sircar), which issued May 11, 1982; and 4,263,018 (McCombs et al.), which issued Apr. 21, 1981.
Pressure swing fractionation processes are commercially attractive for several reasons: simplicity of structure and function, low initial and maintenance costs, and no requirement for substantial power input during operation. However, the conventional pressure swing processes are less efficient than other alternatives. Several factors contribute to this inefficiency. The most significant is the fact that the energy for the process is derived from the pressurized stream itself which is at best a more costly source of energy.
To better understand the inefficiencies of the conventional pressure swing processes, it is helpful to envision the properties of an ideal system. The present inventor recognized that the ideal system would include: an ideal flow with no pressure drop in the beds or plumbing, and an ideal purge in which the partial pressure of the contaminants in the purge stream as it exits the low pressure bed equals the partial pressure of the contaminants in the mixed stream as it enters the high pressure bed. In such an ideal system, the fraction of the incoming stream required for purge would equal the ratio of the absolute pressure of the purge stream to that of the high pressure stream in their respective beds.
In contrast with the ideal system, the ratio of purge to input flow rates in known pressure swing systems is constrained by the fact that the absolute pressure of the purge stream is 14.7 psia (or atmospheric pressure). As the energy which is required to regenerate in conventional processes is essentially the energy required to elevate the purge stream to the higher pressure required for initial purification, it is clear that the operating efficiency of conventional pressure swing systems is constrained from significant improvement. That is, the efficiency of conventional systems is destined by theory and practice to be significantly less than alternative methods of fractionation because the purge stream is constrained to atmospheric pressure.
U.S. Pat. No. 4,557,735 (Pike) discloses an efficient integrated system wherein process energy is utilized efficiently. However, the integrated system disclosed by Pike is extremely complicated and is primarily directed to the preparation of feed air, i.e., compression, cooling and cleaning of the air, for eventual separation by cryogenic rectification. More important, however, Pike pertains to a heat-reactivatable type of adsorbent gas fractionator, not a pressure swing or heatless type such as that disclosed in the present invention. Furthermore, the gaseous mixture is passed through a cryogenic rectification for the purpose of separating the feed air into nitrogen-rich and oxygen-rich components before being warmed and returned to a second purifying heat regenerable adsorbent.
It is an object of the present invention to provide a more efficient pressure swing or heatless adsorbent fractionation process and system for the fractionation of gaseous mixtures PG,9 without the need for introducing additional external energy to the system.
It is a further object of the present invention to take advantage of energy in the form of elevated temperatures in the high pressure mixed stream that may be present as a result of an upstream compression process to enhance the efficiency of the fractionation process.
Another object is to utilize the energy laden in the high pressure purge stream prior to expansion to increase the efficiency of the fractionation process.
The present invention also provides many additional advantages which shall become apparent as described below.